1. Field of the Invention
The invention relates to an attenuating filter providing a prescribed attenuation of the intensity of transmitted ultraviolet light over a predefined wavelength range according to a predefinable spatial distribution of its spectral transmittance.
2. Description of the Related Art
Attenuating filters of the type usually have a plane-parallel substrate fabricated from a material that has a sufficiently high transmittance for ultraviolet light over the wavelength range to be involved, as well as at least one filter coating that provides the desired spatial distribution of spectral transmittance applied to a surface of the substrate. Attenuating filters of the type are employed for, e.g., maintaining constant irradiation levels during long-term endurance tests employing laser light in order to quantitatively assess the abilities of certain samples, such as various types of quartz glass, to withstand irradiation by laser light. Attenuating filters may also be employed for effecting controlled reductions of irradiation levels to defined levels for irradiating samples, conducting calorimetric absorption measurements, or for implementing other methods for reducing or controlling irradiation levels.
One application where accurate maintenance of a prescribed distribution of spatially varying spectral transmittance is of particular importance is the microlithographic fabrication of semiconductor devices or other microelectronic devices. As is well-known, such applications involve the employment of wafer steppers or wafer scanners that, among other things, require minimizing departures from uniform-intensity illumination over the image plane of a projection lens, where specified departures as low as ±2% are commonplace. However, the specified departures are frequently not directly attained in the case of given illumination systems and given projection lenses. In order to eliminate departures from uniform-intensity illumination exceeding tolerable levels, an additional attenuating filter that has a suitably shaped spatial-transmittance profile that will compensate for any variations in irradiation intensity and, preferably, a low reflectance, is inserted into the optical train, immediately ahead of the object plane (reticle plane) of the illumination system involved.
In the case of the aforementioned applications, the optical properties of the filters employed should remain unchanged and the filters should exhibit no noticeable degradation over their service lives. Compliance with this requirement will become increasingly difficult as the wavelengths involved become shorter and the irradiation intensities involved increase. For example, in the case of the latter transmission filters for compensating for variations in irradiation intensity, platinum is employed as the absorbing material of the filter coating on microlithographic projection illumination systems designed for use at wavelengths of 436 nm, 365 nm, and 248 nm. However, platinum exhibits intolerably high degradation at, e.g., wavelengths of 193 nm and shorter wavelengths. In the case of the aforementioned long-term endurance tests wavelengths conducted at wavelengths of 248 nm and shorter wavelengths, solid neutral-density filters having, e.g., absorbing aluminum films like those employed for attenuating light in conjunction with rapidly concluded beam-profile measurements, are employed. However, aluminum films are unsuitable for long-term use, since they oxidize within a few minutes.
Known, multilayer dichroic filters consisting of dielectric-film stacks that have been dimensioned such that the laser wavelength involved lies exactly at their “cut-on” wavelength are employed for longer-duration irradiation-resistance tests. Altering the angle of incidence on same by tilting their substrate relative to the laser-beam axis will lengthen the effective path length for radiation transiting their multilayer dielectric-film stack, which will, in turn, allow altering their spectral transmittance in a controlled manner. However, one disadvantage of that approach is that the spectral transmittance of such filters is extremely sensitive to even the slightest variations in the properties of their multilayer stacks due to the steep gradient in their spectral transmittance in the vicinity of their “cut-on” wavelength. For example, localized de-tuning of their multilayer stacks, such as drifting of their “cut-on” wavelength due to temperature variations, absorption of water by their dielectric films, or accumulations of contamination thereon, may occur after certain periods of irradiation, causing a beam profile to become burned into them, making them unsuitable for further use as spatially homogeneous attenuating filters. Multilayer dielectric filters of the type whose filtering effects are adjusted by tilting them are unsuitable for use as transmission filters to be installed in the vicinities of the reticle planes of projection illumination systems.